Heat exchangers are widely used in many processing operations. One type of heat exchanger in wide use is generally referred to as a shell and tube heat exchanger. Such a heat exchanger has at least one tube extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet as that a suitable heat exchange fluid can be circulated through the shell to cool or heat as a fluid flowing through each tube. Each end of the shell can be left open for use in some processing operations and for others one or both ends of the shell can be closed, such as by a removable cover. Closing of one end of the shell to provide an enclosed feed box space is quite common. In addition, when the heat exchanger is to be used at high temperatures, the other shell end is usually also closed to provide a fluid collecting box to which fluid exits after flowing through each tube. Of course, the feed box is provided with a suitable feed inlet and the collecting box is provided with an outlet.
Although shell and tube heat exchangers are generally used to heat a liquid stream, they are also useful for cooling such a stream. When used for cooling purposes, each tube outlet end can be closed, or it can also be left open or uncovered so that the effluent can exit unrestrictedly into a suitable receptacle. Similarly, the tube inlet end can be enclosed or it can be left open and the liquid to be cooled fed to the tube by any suitable means. Thus, a wier can be provided around the tube sheet so that a pool of liquid is formed and flows into the open mouth of each tube.
Shell and tube heat exchangers of the described types can be used for producing fresh water from brackish water and sea water, for concentrating fruit and vegetable juices, and in industrial crystallization processes. As the liquid flows through each tube it can be cooled enough to crystallize out a solid from the liquid. Thus, by cooling sea water, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed thereby providing a concentrated juice.
The copending United States patent applications of Engdahl et al Ser. No. 160,112 filed June 16, 1980 and now U.S. Pat. No. 4,286,436 issued Sept. 1, 1981 and Engdahl Ser. No. 160,002, filed June 16, 1980 and now U.S. Pat. No. 4,314,455 issued Feb. 9, 1982, disclose heat exchangers (also called freeze exchangers) for cooling liquids. The disclosure of those patent applications is incorporated herein by reference.
The tubes of freeze exchangers, which are a species of heat exchanger, must have a surface which discourages ice from sticking to the tube. Stainless steel tubes with highly polished surfaces are suitably used in freeze exchangers. However, if the polished surface deteriorates to such an extent that ice sticks there could be a loss of efficiency and ultimately plugging of the tube.
Shell and tube heat exchangers (including freeze exchangers) are manufactured with the tubes permanently joined to the tube sheets by welding or by a tube expansion method. Permanent installation of the tubes makes it very difficult and expensive to repair a heat exchanger if a tube leaks, corrodes or becomes plugged. Generally, removal of a tube involves destruction of the tube, and sometimes destruction of adjacent tubes to provide access. In addition, damage to the tube sheets often results.
It has been proposed to use undersized tube sheets with oversized holes and to secure the tube sheets in the shell, and the tubes in the holes, with a solid polymeric material, such as an epoxy resin. Removal of the tubes and tube sheets is to be achieved by applying heat to the polymeric material to melt or degrade it. A heat exchanger fabricated in this manner, however, may not qualify under accepted engineering standards and codes pertaining to heat exchangers because of the present insufficient performance data on solid polymeric materials for the conditions they will be subjected to during heat exchanger operation. If the polymeric material securing the tubes in place in the tube sheets were to fail, and the tubes slip, the restraining force exerted by the tubes to the tube sheets, to which they are connected at each end, would be lost. All the pressure of the heat exchange fluid on the shell side would then be exerted against the tube sheets and only the polymeric material binding it to the shell would be available to resist the pressure. Because the behavior of the polymeric materials is not thoroughly known, a sudden and dangerous tube sheet failure could result.
A need accordingly exists for tube and shell heat exchangers having readily removable tubes secured in place by tube sheets which function as internal structural members, and desirably meet ASME codes and other safety standards.